1. Field of the Invention
The present invention relates generally to the formation of internally ribbed or rifled tubes and, more particularly, to plug tools for forming internal helical or spiral ribs in tubing, and to methods of producing internally helically or spirally ribbed or rifled tubes.
2. Brief Discussion of the Related Art
Internally ribbed or rifled tubes are used for various heat transfer or heat exchange applications including steam generating and superheating equipment such as boilers. Internally ribbed or rifled tubes present various advantages over plain bore tubes including the ability to sustain high heat transfer rates at higher pressures, higher enthalpies and lower mass fluxes than plain bore tubes.
The advantage of using internally ribbed or rifled tubes in furnaces of boilers was recognized during the development of steam boilers. Initially, however, internally ribbed tubes were lap-welded tubes. Eventually, it was proposed to produce seamless internally ribbed tubes for boiler applications as represented by U.S. Pat. No. 1,465,073 to Davis. The Davis patent referred to the internal ribs as increasing the heating surface and the effectiveness of heat transfer to water and steam inside the tubes. The use of internally ribbed tubes was still limited because the benefit derived from the internally ribbed tubes was not as great in many boiling ranges with certain arrangements and dimensions of the ribs.
In a boiler there are two known types of boiling that can occur: nucleate and film. In nucleate boiling, steam bubbles are formed and released on the inside surface of a tube with water wetting the inside surface, whereas in film boiling the inside surface of the tube is covered by a film of steam. Nucleate boiling creates excellent heat transfer conditions as the steam bubbles generated at nucleation points are rapidly detached from the water and move into and agitate the bulk liquid. In film boiling, heat transfer is hindered by the steam film that prevents water from wetting the inside surface of the tube and absorbing heat. U.S. Pat. No. 3,088,494 to Kotch et al. proposed the theory of increased heat transfer in nucleate boiling through an improved internally ribbed vapor generating tube for sub-critical pressure that promoted the maintenance of nucleate boiling irrespective of the positioning of the tube. U.S. Pat. No. 3,213,525 to Creighton et al., U.S. Pat. No. 3,289,451 to Kotch et al., and U.S. Pat. No. 3,292,408 to Hill relate to apparatus and methods for forming internally ribbed tubes of the type disclosed in the Kotch et al. patent. The phenomenon of the change of heat transfer from nucleate boiling to film boiling is also observed at super-critical pressures even though the physical distinction of the steam bubbles and steam film from water by a clear interface or surface is not present and is replaced by steep density and enthalpy gradients. The deterioration in the heat transfer is generally referred to as the boiling crisis or worsened heat transfer to encompass both sub and super-critical conditions.
Boilers are either natural circulation, assisted circulation or once-through in type. All of these can operate at subcritical steam conditions but only once-through boilers offer the possibility of operation at supercritical steam conditions. Ribbed tubing has been applied over the years in all of these wherever the likelihood of worsened heat transfer is perceived, though, in general, the assisted circulation and once-through technologies do not require it because the tube internal flow rates can be sufficient to maintain high heat transfer rates. Once-through steam generators having internally ribbed tubes forming a vertical gas flue or combustion chamber are disclosed in U.S. Pat. No. 5,662,070, No. 6,735,236, No. 5,967,097, No. 6,250,257 B1 and No. 6,302,194 B1 to Kastner et al, and in European Patent EP 0 581 760 B2.
Ribbed tubing exists in many forms, from the earliest lap welded varieties, through seamless variants such as single lead ribbed tubes and multi-lead rifled tubes. The heat transfer properties of this “standard” ribbed tubing allows the mass flux within the tubes to be reduced when compared to plain bore tubing. To date, however, it has not allowed sufficient reduction to enable a once-through boiler furnace to be cooled successfully in a single vertical pass. The introduction of “optimized” multi-lead rifled tubing with steeper lead angle and taller rib, as represented by Kastner et al ('194), has finally achieved this goal but the manufacturing difficulties have been such that it could not be economically produced in commercial quantities. Small lengths of rifled tubing produced for performance testing purposes do not reveal the range of problems associated with the manufacture of rifled tubing in commercial quantities. The present invention addresses these problems by changes to the tooling and manufacturing processes. Additionally, modifications to the rib profile have reduced manufacturing forces.
Despite advancements in the production of internally ribbed tubes, the majority of once-through boilers in operation today have furnace walls built with plain bore tubes for heat transfer requirements at high mass flux flow density. Although some once-through boilers employ internally ribbed tubes, they are also usually assigned to operate at high mass flux flow density. The Kastner et al patent ('194) discloses internally ribbed tubes designed for use independently of mass flow rate density, thereby allowing higher steam contents and reducing undesired temperature increases in the tube wall. Production of internally ribbed tubes pursuant to the Kastner et al. patent presents fabrication problems such as difficulty in removing the rib-forming tool, the presence of stresses that cause frequent tool breakages, and a very slow rate of production.
Internal helical or spiral ribs may be formed in tubing via a cold drawing process in which the tubing is drawn longitudinally over a plug tool comprising an externally helically or spirally grooved plug disposed in a die orifice of a drawing die. The drawing die constricts the tubing as it is drawn through the die orifice, causing the internal surface or wall of the tubing to be forced into the external grooves of the plug. The plug rotates about its central longitudinal axis within the die orifice as the tubing is drawn through the die orifice over the plug. Accordingly, continuously extending helical or spiral ribs closely corresponding to the external grooves of the plug are formed in the internal wall of the tubing. The plug is typically coupled with a shaft or back-bar of the draw bench, with the tubing being advanced over and along the shaft or back-bar as it enters the die orifice.
In addition to the patents referred to above, illustrative apparatus and methods for forming internal ribbing in tubing are represented by U.S. Pat. No. 2,358,838 to Wadell, U.S. Pat. No. 2,852,835 to Harvey et al., U.S. Pat. No. 3,768,291 to Reiger, U.S. Pat. No. 4,733,698 to Sado, U.S. Pat. No. 4,847,989 to Franks, U.S. Pat. No. 4,854,148 to Mayer, U.S. Pat. No. 4,866,830 to Zoller, U.S. Pat. No. 4,876,869 to Saki et al., U.S. Pat. No. 4,921,042 to Zoller, U.S. Pat. No. 4,938,282 to Zoller, U.S. Pat. No. 4,942,751 to Fuchs, Jr., U.S. Pat. No. 5,010,643 to Zoller, U.S. Pat. No. 5,690,167 to Reiger, U.S. Pat. No. 6,302,194 to Castner et al., and U.S. Pat. No. 6,488,078 to Buetler et al., and by U.S. Patent Application Publication No. US2003/0094272 to Brand et al.
Many prior apparatus and methods for forming internal helical or spiral ribbing in tubes are associated with drawbacks due to the forces imposed on the plug tool during the drawing process leading to relatively short tool life, relatively low productivity and the inability to economically and consistently produce internally helically or spirally ribbed tubes in commercial quantities. Internally helically or spirally ribbed tubes produced using prior apparatus and methods may present deficiencies when used in a vertical arrangement in a once-through boiler including undesirable boiler feed pump power requirements, flow characteristics and furnace temperatures, as well as greater complexity and cost of construction.